This invention relates to "custom integrated circuits" made by a method whereby standard integrated circuits, each containing an array of transistors and resistors, is made in advance and interconnections of these circuit components is thereafter made quickly to conform with a customer's special circuit specification.
In an integrated circuit, a system of metal films provide termination pads as well as interconnective means between the components that may be formed in or on a semiconductor die. Large numbers of identical integrated circuits are formed on a single crystalline semiconductor wafer by a series of process steps most of which add structure to some part of all the integrated circuits on the wafer simultaneously. Such is the case for most of the steps normally employed to form the system of interconnective and terminating metal films in each integrated circuit.
Such a system of metal films is normally deposited after forming at one surface in the wafer the diffused portions of transistors and diffused resistors. A glass layer is formed over the wafer surface. Holes are provided in the glass layer through which electrical access may be had to some or all of the elements of each transistor and to the contact areas of resistors. Aluminum is usually vacuum deposited over the glass, forming a single aluminum film over the entire wafer surface making contact through the holes in the glass with the transistors and resistors.
A photo-resist layer is applied over the single aluminum film. The photo-resist film is exposed to light in regions that are to be selectively removed, or vice versa, by a subsequent etch step so as to form the above mentioned metal patterns on each integrated circuit. This exposure to light of the photo-resist is usually accomplished by a standard sequential step-and-repeat method whereby the photo resist layer over each integrated circuit region is one at a time exposed to a pattern of light corresponding to the ultimate metal system pattern desired for each of the identical integrated circuits.
This light pattern is obtained by preparing artwork at a greatly enlarged scale, optically reducing the scale and photographing the artwork to form a photo-mask (transparency) and exposing the photo resist layer on the wafer through that photo-mask.
The artwork is typically a composite sheet consisting of an actinically opaque top film bonded lightly to a transparent temperature stable sheet, e.g. Mylar. By a standard cut-and-strip technique, the opaque film is cut so that portions of it may be peeled off and removed to form the desired pattern. Programmable machines have been developed to perform this cut-and-strip operation. Whether the cutting and stripping is done manually or by machine, a very accurate drawing of the desired pattern must be prepared by an engineering draftsman. Many man hours are usually required to make such drawings.
It is an object of this invention to provide a means by which a manufacturer may make integrated circuits to a customer's specifications on a custom service basis which method is simpler, faster and more economical.
It is a further object of this invention to provide such a method whereby a standard integrated circuit includes an array of transistors and standard but partially completed cut and strip artwork is provided which may be easily completed by the customer to avoid the need for the aforementioned draftman's attention.